1. Field of the Invention
The present invention relates to a variable impedance circuit, a variable gain differential amplifier using the same, a multiplier and a differential distributed amplifier using the same and a high-frequency circuit using them.
2. Description of the Background Art
Variable gain differential amplifiers (differential amplification circuits having a variable gain function) have been conventionally in use. Integrated circuits using Si (silicon) devices, such as bipolar transistors and MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), mainly include an amplifier having the Gilbert-cell structure or OTA (operational transconductance amplifier) structure as a variable gain differential amplifier.
An amplifier having the Gilbert-cell structure has a wide variable gain range, but is inferior in power consumption and noise characteristics. For this reason, mobile communication devices and the like in general use the OTA structure having a variable resistance circuit formed by an FET switch and the like in a differential amplifier.
FIG. 35 is a circuit diagram of a conventional variable gain differential amplifier having the OTA structure.
The variable gain differential amplifier shown in FIG. 35 is formed by bipolar transistors (hereinafter abbreviated to transistors) 101 and 102, resistors 103, 104, 105 and 106, and an n-MOSFET (hereinafter abbreviated to an FET) 107. The FET 107 forms a variable resistance circuit 50.
The base of the transistor 101 is connected to an input terminal NI1 receiving an input signal RFin(+), and the base of the transistor 102 is connected to another input terminal NI2 receiving another input signal RFin(−). The input signals RFin(+) and RFin(−) are differential inputs. The collectors of the transistors 101 and 102 are connected to a power supply terminal NVC receiving a power supply voltage Vcc via the resistors 103 and 104, respectively. The emitters of the transistors 101 and 102 are connected to ground terminals via the resistors 105 and 106, respectively. The collectors of the transistors 101 and 102 are connected to output terminals NO1 and NO2, respectively. Output signals RFout(−) and RFout(+) are derived from the output terminals NO1 and NO2, respectively. The output signals RFout(+) and RFout(−) are differential outputs.
The FET 107 is connected between nodes N1 and N2 that are connected to the respective emitters of the transistors 101 and 102. The gate of the FET 107 is connected to a control terminal NG receiving a control voltage AGC via a resistor 110.
In the variable gain differential amplifier shown in FIG. 35, the control voltage AGC is applied to the gate of the FET 107 for varying a source-to-drain resistance of the FET 107 so as to perform gain control. The maximum gain and lowered noise characteristics can be achieved when the FET 107 is in an ON-state, for example. In this case, the variable gain differential amplifier is suitable for amplifying a small high-frequency signal. On the other hand, the amount of attenuation is maximized (minimum gain) to enhance distortion characteristics when the FET 107 is in an OFF-state. In this case, the variable gain differential amplifier is resistant against cross modulation in a state of high electric field strength.
Also for Gilbert-cell multipliers, a similar structure as that of a variable gain differential amplifier having the OTA structure has been proposed.
FIG. 36 is a diagram showing the structure of a conventional differential input/output high-frequency circuit used for a high-frequency receiver.
The differential input/output high-frequency circuit shown in FIG. 36 is formed by a variable gain amplifier 610, multiplier 620, and a variable gain intermediate frequency band multiplier (hereinafter abbreviated to an IF band amplifier) 630. A differential signal is input to the variable gain amplifier 610, and an amplified differential signal is output from the variable gain IF band amplifier 630. The variable gain amplifier 610 and the variable gain IF band frequency 630 are supplied with a control voltage AGC for controlling the gain.
The variable gain amplifier 610 is formed by a differential amplifier having a variable gain function; the multiplier 620 is formed by a Gilbert-cell amplifier without a variable gain function; and the IF band amplifier 630 is formed by a differential amplifier having a variable gain function.
Thus, in the differential input/output high-frequency circuit, the dynamic range of a differential amplifier used for the first stage greatly affects the dynamic range of a high-frequency receiver. In this case, the operating conditions of the Gilbert-cell multiplier are not optimized, resulting in a small dynamic range.
For this reason, receivers having a high-frequency amplifier, a mixer, and an intermediate frequency detecting circuit provided with an AGC (Automatic Gain Control) circuit that controls the gains of the high-frequency amplifier and the mixer have been proposed (refer to, for example, JP-5-300039-A.)
Distributed amplifiers are well known amplifiers that operate over a frequency band of one octave or wider in microwave band to millimeter wave band (refer to, for example, JP-9-252228-A, JP-11-88079-A, and JP-2003-209448-A.)
FIG. 37 is a circuit diagram showing one example of the structure of a conventional distributed amplifier. The distributed amplifier has a plurality of transistors TR1 to TR4, the gates (input terminals) of the plurality of transistors TR1 to TR4 being connected via respective inductive elements IL1 to IL4 each formed by a high-impedance transmission line or inductive element, the drains (output terminals) of the plurality of transistors TR1 to TR4 being connected via respective inductive elements OL1 to OL4 each formed by a high-impedance transmission line or inductive element. The parasitic capacitances of the respective transistors TR1 to TR4 (gate-to-source capacities on the input side and source-to-drain capacities on the output side) and the inductances IL1 to IL4, OL1 to OL4 thus form a quasi-transmission line. As a result, input and output impedance matching over a wide band can be achieved. In general, distributed amplifiers having a larger number of stages of transistors operate in a wider band.
In the variable gain differential amplifier shown in FIG. 35, however, the variable resistance circuit 50 has strong non-linearity at a region of the control voltage around a pinch-off voltage for the FET. This causes deterioration in the distortion characteristics around a specific control voltage. In the case of continuous gain control, therefore, the distortion characteristics of the variable gain differential amplifier are deteriorated when the FET is supplied with a control voltage at which waveform distortion is increased.
It is possible to increase emitter resistances of the transistors 101, 102 to achieve improved distortion characteristics in the variable gain differential amplifier. In this case, however, the operating current of the variable gain differential amplifier varies with varying gain.
In some variable gain differential amplifiers, it is desired to improve the distortion without variation of the operating current, depending on their uses.
Further, in the variable gain differential amplifier shown in FIG. 35, when the input voltage level is constant, the output voltage level increases at higher gain to easily cause saturation of the input/output characteristics. This is because the operating current of the variable gain differential amplifier does not vary with varying gain.
In some variable gain differential amplifiers, it is desired to set the operating current according to the output voltage level to suppress the saturation of input/output characteristics, depending on their uses.
In addition, in a conventional receiver using the variable gain differential amplifier shown in FIG. 35, a sufficiently high dynamic range has not been realized.
Distributed amplifiers are used in high-speed digital signal transmission systems, for example. In the high-speed digital signal transmission system, an amplifier having a variable gain function is effective in varying the gain in case of variation in the input level.
For this reason, a structure of a distributed amplifier has been proposed in which cascode-connected transistors are used in each of amplifying sections, and the transistors are individually turned on/off (refer to, JP-2003-298370-A, for example.)
In the structure in which the transistors are individually turned on/off, however, the gain only varies in a discrete manner.